- For LC/MS applications: Positive ion mode favors acidic mobile phases and Negative ion mode favors basic mobile phases. However, feel free to experiment using both ionization modes and don't forget about using adducts (e.g. ammonium and sodium) with all types of samples to improve signal response. *Maintain these buffers at or below 10 mM. Adjust the pH of the mobile phase to be 1 to 2 units away from your sample's pKa.
Table 1: Popular examples of useful volatile mobile phase buffering systems.
BUFFERING AGENT USEFUL pH RANGE
- Ammonium formate 2.8 - 4.8; 8.2. - 10.2
- Formic Acid 3.3 - 4.3
- Pyridine/Formic Acid 3.3. 4.3, 4.8 - 5.8
- Trimethylamine/Formic Acid 3.3 - 4.3, 9.3 - 10.3
- Ammonium Acetate 3.8 - 5.8; 8.2 - 10.2
- Acetic Acid 4.3 - 5.3
- Trimethylamine/Acetic Acid 4.3 -5.3, 9.3 - 10.3
- Ammonia/Formic Acid 3.3 - 4.3, 8.8 - 9.8
- Ammonia/Acetic Acid 4.3 - 5.3, 8.8 - 9.8
- Ammonium Bicarbonate 5.9 - 6.9, 8.8 - 9.8
- Ammonium Carbonate 5.9 - 6.9, 8.8 - 9.8
- Carbonic Acid 6 - 8
- 1-Methylpiperidene 10.0 - 12.0
*Notes: (1) Formic acid (3.75) is slightly stronger and more volatile than Acetic acid (4.75). Formic acid is often available in higher purity grades and absorbs less in the UV region making it a better choice for most chromatography applications. It works well in positive mode LC/MS analysis, esp at 0.1%. (2) Trifluoroacetic acid (TFA, pKa = 0.3) is very strong and volatile, but we do not recommend its use in LC/MS applications as it can increase the background signal levels (esp. in Negative Mode) LC/MS (m/z 113), be very hard to remove from the source and result in long term contamination.(3) Triethylamine (TEA, pKa 11) is volatile, strong and very stable, but causes similar contamination problems resulting in high background signals when used in Positive Mode LC/MS (m/z 102). (4) Many ion-pairing reagents suppress ionization, bind to the plastics and metals used and contaminate the flow path. If you must use them, please do so using the lowest concentrations levels possible and thoroughly decontaminate the entire system after use. Minimize further contamination by labeling and using a dedicated column for the application (Do not use that column for other applications).